1. Field of the Invention
The present invention relates to a data and reproduction apparatus which is called a data streamer.
2. Description of the Related Art
In a data recording method employing a magnetic tape as a recording medium, what is commonly called a non-tracking method is known.
As shown in FIG. 1, a magnetic tape T is wound around a rotating head, and tracks TK are formed obliquely with respect to the direction of the movement of tape. The non-tracking method is designed so that, by performing scanning at a density higher than that during reproduction as indicated by a solid line Pa and a dotted line Pb, all data on the tracks TK can be read without accurately tracing on the tracks, and by rearranging the read-in data by using the address recorded together with the data, a reproduction data stream can be reconstructed accurately.
FIG. 2 shows the structure of the tracks TK on a magnetic tape in the non-tracking method.
As shown in FIG. 2, one track is made up of 108 blocks, and one block is composed of 288 bits.
The 92 blocks in the central portion of the track are made to be a main data area, and an inner double recording area of 9 blocks and an outer double recording area of 7 blocks are formed on both sides of the main data area.
In the inner double recording area, data having the same content as that of the block in the main data area spaced 92 blocks away in the outward direction from the position of the inner double recording area is recorded. In the outer double recording area, data having the same content as that of the block in the main data area spaced 92 blocks away in the inward direction from the position of the inner double recording area is recorded. In other words, as shown in FIG. 2, data having the same contents as that of the leading 7 blocks (the shaded portion) of a data area 1 is recorded in the outer double recording area at a different address, and data having the same contents as that of the trailing 9 blocks (the dotted-line portion) of the data area 1 is recorded in the inner double recording area at a different address. Therefore, even if the touch position of the head is deviated due to fluctuation of tape, the data is designed to compliment each other. That is, blocks (data contents) which cannot be read, in particular, at the leading and trailing ends, will not occur for the data in the form of blocks which are recorded within the main data area.
The two central blocks of the main data area are allocated to an area for subcodes (AUX), each one block on both sides thereof is allocated to an area for IBG (Inter-Block Gap), and each four blocks on both sides thereof are allocated to an area for control codes (CTL). Further, on both sides thereof, data areas of 40 blocks are formed.
The signal format within one block is as shown in FIG. 2. The leading 11 bits are made to be a sync pattern, and an address ADRS is recorded by the subsequent 13 bits. The address ADRS is made up of a track address of 6 bits and a block address of 7 bits. Since a track address and a block address are recorded in each block in this manner, it is possible to reconstruct a data stream in an appropriate block sequence during reproduction.
In the case of a non-tracking method, since tracks TK are not always traced accurately, all blocks can be read out for each track by performing high-density scanning as shown in FIG. 1. In this case, however, the reading sequence of each block is random. The read block data is temporarily stored in a RAM. At this time, a writing address is created by using the track address and the block address in the RAM, and each block of data is written. Therefore, at the stage where all the blocks are read for a certain track, all the data of that track is arranged in the RAM. Therefore, if block data is read out in sequence from the RAM, an appropriate data stream is reconstructed.
Following the address ADRS, P and Q parities (P.sub.OD, Q.sub.OD, P.sub.EV, Q.sub.EV) each comprising 4 words are each recorded by 12 bits per word. Following the parity words, data comprising 16 words (DT.sub.1 to DT.sub.16) are each recorded by 12 bits per word. Following the 16-word data (DT.sub.1 to DT.sub.16), two CRCs (Cyclic Redundancy Check Codes) words are each recorded by 12 bits per word. An overwrite protect code (hereinafter referred to as an "OWP code") is recorded in the CRC word.
In the non-tracking method, since a deviation of a recording area is allowed, old data might be left without being erased near both ends of the track. Also, there might occur unerased portions which are not erased at overwrite time due to omission during recording or the clogging of the head. Since such unerased data is safe in terms of CRC during reproduction, the data is incorrectly recognized as correct data. Therefore, an OWP code is recorded as a code which is updated at each pause in the recording operation.
During reproduction, an OWP code is extracted from each block read out for a track to be scanned for reproduction, and a reference OWP code is set by a majority decision. In a case where an unerased portion has occurred in a certain portion within one track, the OWP code extracted from the unerased block is different from the OWP code which is extracted from the overwritten block. Therefore, when a track is reproduced, most correctly overwritten blocks can be read even if there is a partially unerased portion. Therefore, by deciding an OWP code by majority, the OWP code at the majority side can be determined as an OWP code set when it is correctly overwritten.
The above-mentioned OWP code is set as a reference OWP code. Thereafter, during reproduction for the series of records, data of the block having a different OWP code is determined to be unerased data, and the data can be nullified. Thus, it is possible to prevent erroneous data from being output.
The OWP code is recorded after Exclusive-OR is computed with the CRC of 24 bits in which the same two words are arranged. Therefore, during reproduction, it is possible to reconstruct the OWP code by computing Exclusive-OR with the CRC created from the reproduced data.
When recording data, such as computer programs, on a magnetic tape is taken into consideration, omission of data and recording of erroneous data during recording must be avoided. For this reason, the recorded data is checked after the data is recorded (check-after-write).
For this check, as heads to be disposed, for example, in a rotating drum, heads A.sub.1 and B.sub.1 are disposed, and heads A.sub.2 and B.sub.2 are disposed at positions oppositely facing the heads A.sub.1 and B.sub.1 by 180.degree.. Data is recorded in the form of tracks by the heads A.sub.1 and B.sub.1, and the data of the recorded tracks is reproduced by the heads A.sub.2 and B.sub.2 so that a check is made to determine if the data has been correctly recorded. The heads A.sub.2 and B.sub.2 trace the recorded tracks with a difference of several tracks with respect to the heads A.sub.1 and B.sub.1.
Here, when recording by the heads A.sub.1 and B.sub.1, the track address (ADA-V) is of a repeat value of 0 to 31, and the track address is incremented per one rotation of the drum, and is recorded in the address ADRS of each block of FIG. 2.
When, for example, there is a previously recorded data file on the tape T as shown in FIG. 3A, track address (ADA-V) 0 to 31 shown in FIG. 3B is repeatedly recorded for each track constituting the data file. The track address (ADA-V) also serves as an address of RAM for temporarily storing data during recording and reproduction as described above. When the RAM has a capacity for 32 tracks, a value from 0 to 31 is set in the track address (ADA-V).
It is now assumed that, for example, following the previously recorded data file, a new data file is begun to be recorded from the position indicating the start of recording as shown in FIG. 3A. If it is assumed that the track address (ADA-V) of the final track is sixteen when recording of the previously recorded data file stopped, a new data file is begun to be recorded starting at the track address (ADA-V) of 17. Then, the track address (ADA-V) is updated as 18, 19, . . . , 31, 0, 1 for each track.
In a case where recording by the heads A.sub.1 and B.sub.1 starts, the heads A.sub.2 and B.sub.2 which trace with a difference of several tracks will at first trace a previous data file. Therefore, the heads A.sub.2 and B.sub.2 trace data which is not related to the data of the track recorded by the heads A.sub.1 and B.sub.1, and thus the data read by the heads A.sub.2 and B.sub.2 is not necessary for checking data.
However, since, in practice, the point at which the tracks recorded for this time start cannot be determined from the reproduction data, a data check operation based on data read by the heads A.sub.2 and B.sub.2 must be also performed before the leading track on which recording is actually started as a data file for this time is reached. The data check operation for the previously recorded data tracks wastes electric power, and in some cases an improper check operation might be performed.
In a case where a series of data files are recorded on the magnetic tape T as described above, an OWP code is added; for example, for tracks which constitute file X as shown in FIG. 4A, an OWP code is recorded as OWP.sub.x in each block. Also, for tracks which constitute file Y, an OWP code is recorded as OWP.sub.y in each block.
When, for example, file X is reproduced, a reference OWP code is set in OWP.sub.x, the reference OWP code is compared with the OWP code for each block. When the reference OWP code matches the majority reference OWP code, the block is assumed to be valid data. Even if reproduction scanning is performed in the direction indicated by the long dashed line P in FIG. 4A and the block of file Y is read in, the block is set as OWP.sub.y and OWP.sub.y is different from the reference OWP code OWP.sub.x, and the block is not assumed to be valid data of file X. Thereafter, when the process proceeds to the reproduction of file Y, the reference OWP code is changed to OWP.sub.y because a greater number of OWP codes which become OWP.sub.y are read in, and the blocks of the file Y are assumed to be valid data.
It is now assumed that data file Z is overwritten on file X as shown in FIG. 4B and the OWP code in this case is OWP.sub.z. It is assumed that, however, an unerased portion occurs due to some reason, such as the clogging of the head, and previous data of file X is partially left as indicated by the shaded portion REC-ER in FIG. 4B.
If this data is reproduced, regarding data read out by the reproducing head from the magnetic tape T, unerased data D.sub.x which constitutes file X is contained in the middle of data D.sub.z which constitutes file Z as shown in FIG. 4C. The OWP code of the block having data Dx is OWP.sub.x as shown in FIG. 4D. Since the reference OWP code which is set during reproduction as shown in FIG. 4E is set to OWP.sub.z at first, and it is compared with the OWP code (OWP.sub.z) of the block having data D.sub.z, data D.sub.z is made to be valid reproduction output when a code match occurs.
However, when the process proceeds to the reproduction of the unerased portion, a great number of OWP.sub.x is read out, the reference OWP code is updated to OWP.sub.x as shown in FIG. 4F. After the reference OWP code is changed, the unerased data Dx which is read out is processed as valid data. Since a great number of OWP.sub.z is read out again after the unerased portion is reproduced, the reference OWP code is set to OWP.sub.z again, and data D.sub.z which is read out is made to be valid reproduction output.
With the above-described operation, in the reproduction of file Z, unnecessary data might be made to be valid data in a period ED.sub.1 as shown in FIG. 4F, and valid data which is read out in a period ED.sub.2 might be omitted, causing a serious problem that the reliability is decreased.